Engine performance diagnostic procedures are covered in "Engine/Emissions Controls" service manual and should be consulted for diagnosis of any driveability, emissions, or Service Engine Soon lamp complaints.
| • | Refer to Engine Controls - General Information. |
| • | Refer to Engine Controls - Fuel Injection (MFI). Check for any related service bulletins or service manual updates prior to extensive engine diagnosis. |
Important: The following diagnosis information covers common problems and possible causes. When the proper diagnosis is made, the problem should be corrected by adjustment, repair or part replacement as required. Refer to the appropriate section of the service manual for these procedures.
This diagnostic table will assist in engine misfire diagnosis due to a mechanical problem such as a faulty camshaft or leaking head gasket. Base engine timing is assumed to be correct and, that acceptable fuel is being used. This table will not identify a crossed injector wire, faulty injector, coil, driver, etc., or any other component failure or perceived misfire with no driveability symptoms (e.g. chipped reluctor ring with a DTC set or Service Engine Soon Light illuminated). The On-Board Diagnostic System Check must be performed first in order to ensure the correct table is followed for diagnosis of a misfire. Refer to Engine Controls first, then, if necessary, refer to this table.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Measure the engine oil pressure at the intake camshaft carrier. | |||
Is the oil pressure at operating temperature in specs? | -- | Go to Step 2 | Go to Step 3 | |
2 | Be aware that oil viscosity affects oil pressure. | |||
Is the recommended oil viscosity being used based on the ambient temperature? | -- | Go to Oil Pump Replacement | Go to Step 3 | |
3 | Are there any obvious mechanical noises? | -- | Go to Engine Noise Diagnosis | Go to Step 4 |
4 | Visually inspect valve train and timing components spark plugs, coils, integrated ignition components, and hoses for proper hook up or damage. | |||
Is there any damaged component or improperly connected wires or hoses? | -- | Go to Step 5 | Go to Step 6 | |
5 | Repair or replace the parts as necessary. | |||
Do the symptoms still exist? | -- | Go to Step 7 | System OK | |
6 | Remove and inspect the spark plugs. | |||
Are any spark plugs contaminated with the engine coolant? | -- | Go to Step 15 | Go to Step 7 | |
7 | Are the spark plugs contaminated with engine oil? | Go to Step 9 | Go to Step 14 | |
8 | Check valve stem oil seals and valve guides for wear. | |||
Is the misfire corrected? | -- | System OK | Go to Step 9 | |
9 | Repair or replace valve stem oil seal and/or service cylinder head. | |||
Is the misfire corrected? | -- | System OK | Go to Step 10 | |
10 |
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Do the symptoms still exist? | -- | Go to Step 15 | System OK | |
11 | Remove and inspect the intake manifold and the intake manifold gasket. (Refer to "Intake Manifold (Unit Repair) Removal" in this section.) | |||
Was there any damage noted on the intake manifold or the gasket? | -- | Go to Step 12 | Go to Step 13 | |
12 | Replace the parts as necessary. | |||
Do the symptoms still exist? | -- | Go to Step 15 | System OK | |
13 | Remove and inspect the cylinder head. Refer to "Cylinder Head Clean and Inspect" in this service manual. | |||
Are the valve guides or the seals damaged? | -- | Go to Step 12 | Go to Step 14 | |
14 | Remove and inspect the piston, the piston rings, and the cylinder for wear. | |||
Are the piston components damaged? | -- | Go to Step 12 | Go to Step 13 | |
15 | Perform a cylinder leak test. Does the engine pass the cylinder leakdown test? | -- | Go to Step 16 | Go to Engine Unit Repair |
16 |
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Are both vacuum readings the same? | -- | Go to Step 18 | Go to Step 17 | |
17 | Inspect each vacuum operated accessory/vacuum line and repair any open or leaking components. | |||
Do the symptoms still exist? | -- | Go to Step 19 | System OK | |
18 | Using a spray bottle, lightly spray the ignition module while listening to the engine. | |||
Did the engine stumble or stall? | -- | Go to Step 19 | Go to Step 20 | |
19 | Replace the ignition module or connectors. | |||
Do the symptoms still exist? | -- | Go to Step 21 | System OK | |
20 | Remove the intake manifold and carefully inspect the intake manifold gasket. | |||
Is there any apparent damage or problem with the gaskets or the sealing surface? | -- | Go to Step 21 | Go to Step 22 | |
21 | Repair or replace the damaged components. | |||
Do the symptoms still exist? | -- | Go to Step 22 | System OK | |
22 | Measure the camshaft lobe lift. | |||
Is the camshaft within specifications? | -- | Go to Step 24 | Go to Step 23 | |
23 | Replace the camshaft. Refer to "Camshaft Replacement (Front Cover in Place) In-Vehicle" in this section. | |||
Does the symptom still exist? | -- | Go to Step 24 | System OK | |
24 | If the problem is still suspected to be mechanical, remove the engine and perform a complete disassembly and inspection. | |||
Is the symptom repaired? | -- | System OK | Refer to "Engine/Emission Controls" section of this service manual. |
